Recently, photo-trigger thyristors having a built-in zero-cross circuit using a MOSFET are widely used as an ON/OFF switching element for AC control for use in an home electric appliance, an electronic copying machine or the like. Such a photo-semiconductor device is disclosed in, for example, Published Unexamined Japanese Patent Application No. 59-151463 and its corresponding U.S. patent application Ser. No. 451,792, and U.S. patent application Ser. No. 555,025.
FIG. 1 is a general equivalent circuit of a photo-trigger thyristor with a zero-cross function. A photo-trigger thyristor 11 is ignited by an optical trigger signal. A resistor 12 is connected between the gate and cathode K of the photo-trigger thyristor 11. A current path between the source and drain of a MOSFET 13 is connected in parallel to the resistor 12, or is connected between the gate and cathode K of the thyristor 11. The MOSFET 13 has its back gate connected to the cathode K. A Zener diode 14 has its anode connected to the cathode K and its cathode connected to the gate of the MOSFET 13. A voltage pickup circuit 15 comprising capacitors C1 and C2 serves to apply to the gate of the MOSFET 13 a gate bias voltage corresponding to a voltage applied between the anode A and cathode K of the thyristor 11, and this function is equivalently represented by the capacitors C1 and C2. The capacitor Cl has its one electrode connected to the anode A of the thyristor 11, and the other electrode connected to one electrode of the capacitor C2 and the gate of the MOSFET 13. The other electrode of the capacitor C2 is connected to the cathode K. The capacitor Cl is the capacitance of a PN junction formed to pick up a voltage between the anode A and cathode K of the thyristor 11. The capacitor C2 is the combined capacitance of the capacitance of the depletion layer of the PN junction of the Zener diode 14 and the gate capacitance of the MOSFET 13, i.e., it equivalently represents a parasitic capacitance.
A gate current generated by an optical trigger signal supplied to the gate of the photo-trigger thyristor 11 has a smaller value as compared with the gate trigger current of an ordinary thyristor, so that the thyristor 11 is required to have a high gate sensitivity. With the photo-trigger thyristor 11 designed to have a high gate sensitivity, however, the dV/dt withstandability decreases. Like this, the gate sensitivity and the dV/dt withstandability have a trade-off relation. Accordingly, the MOSFET 13 for controlling the gate sensitivity is provided to improve the relation between the gate sensitivity and the dV/dt withstandability so as to prevent the dV/dt withstandability even for the thyristor 11 with a high gate sensitivity from decreasing.
The MOSFET 13 is in an OFF state when the gate voltage is less than the threshold voltage Vth of the MOSFET 13, and the photo-trigger thyristor 11 can operate with its original high gate sensitivity, i.e., it can be turned on by a minute current such as an optical trigger signal current. When the gate voltage becomes equal to or greater than the threshold voltage Vth, the MOSFET 13 is turned on and the gate and cathode K of the photo-trigger thyristor 11 are short-circuited. With the MOSFET 13 rendered on, the thyristor 11 produces the same effect as a thyristor with the cathode and emitter short-circuited. This reduces the gate sensitivity of the photo-trigger thyristor 11 s that it will not be turned on by a minute optical trigger signal current, thus improving the dV/dt withstandability. In order to perform the above-described operation, the gate of the MOSFET 13 is applied with a voltage proportional to the anode-cathode voltage VAK of the thyristor 11 from the voltage pickup circuit 15. Assuming that the gate voltage of the MOSFET 13 reaches the threshold voltage Vth (=3 V) when the AC voltage VAK becomes 5 V, this photo-trigger thyristor 11 is turned on when given an optical trigger signal current during a phase at which VAK is 0 to 5 V, but is not turned on even when given the optical trigger signal current at the phase of VAK exceeding 5 V. A thyristor whose trigger function works in a specific voltage range at the proximity where an AC voltage applied to the main electrodes (anode and cathode) of the thyristor crosses a voltage of 0 V (the range will be hereinafter referred to as zero-cross portion., 0 to 5 V in the above example) is called a zero-cross type thyristor. A circuit comprising a MOSFET, etc. for providing a zero-cross function is called a zero-cross circuit.
The zero-cross type thyristor has the following two main effects. First, in performing the ON/OFF control of AC power of an commercially available frequency by a thyristor which is not of a zero-cross type, when the thyristor is turned on at the phase of a high value AC voltage (depending on a load), noise generally occurs due to by a rush current or a transient voltage. This would cause a malfunction of an LSI circuit, an IC logic circuit, etc. provided near the thyristor or give an electromagnetic trouble, such as a radio or TV noise problem to electronic appliances. The zero-cross circuit has an effect to significantly suppress the electromagnetic trouble. The second effect is such that with a thyristor designed to have a high gate sensitivity, the high gate sensitivity is given only when the AC voltage VAK is at the phase of the zero-cross portion, and the thyristor has a so-called cathode-emitter short-circuit structure at other phases to thereby significantly reduce the gate sensitivity. As a result, the dV/dt withstandability is improved.
The Zener diode 14 is provided to protect the gate insulating film of the MOSFET 13. This is because the thickness of the gate insulating film of the MOSFET 13 is determined mainly by the desired threshold voltage Vth and cannot be set sufficiently thick to prevent the dielectric breakdown. In other words, the diode 14 having a Zener voltage smaller than the dielectric breakdown voltage of the gate insulating film is provided between the gate of the MOSFET 13 and the cathode K of the thyristor 11, so that when an abnormal voltage exceeding the Zener voltage is applied, it is broken down to be led to the cathode K.
The output voltage of the voltage pickup circuit 15 (voltage at the common node between the capacitors C1 and C2) is substantially equal to the reciprocal ratio of the capacitors C1 and C2 with respect to the anode-cathode voltage VAK of the thyristor 11. When the voltage VAK is small, the capacitance of the depletion layer at the PN junction of the capacitor Cl is very large, and the gate voltage of the MOSFET 13 is substantially equal to the voltage VAK. This gate voltage (the same as the output voltage of the voltage pickup circuit 15) will hereinafter be denoted by VP.
In the above-described zero-cross type photo-trigger thyristor 11 exists a specific characteristic representing the build-up rate dVAK/dt of the maximum anode-cathode voltage VAK for the photo-trigger thyristor 11 to be turned on at the zero-cross portion (this characteristic will be hereinafter called "dV/dt ON characteristic"). When the build-up rate of the anode-cathode voltage VAK becomes equal to or greater than the value of this dV/dt ON characteristic, the gate voltage VP of the MOSFET 13 reaches the threshold voltage Vth before the photo-trigger thyristor 11 becomes completely on, i.e., before the conducting current reaches a latching current, thus turning on the MOSFET 13. Consequently, the photo-trigger thyristor 11 cannot be turned on a minute gate current such as an optical trigger signal current. Such a dV/dt ON characteristic has a strong correction with the voltage VAK (hereinafter called "voltage VW"), at which the MOSFET 13 starts being driven and which is determined by Vth of the MOSFET 13 or VP/VAK or the like. Accordingly, the dV/dt ON characteristic has a trade-off relation with the dV/dt withstandability, one of the basic characteristics of a thyristor. Conventionally, therefore, if the voltage VW is increased to improve the dV/dt ON characteristic, the dV/dt withstandability tends to decrease, and when the voltage VW is reduced to improve the dV/dt withstandability, the dV/dt ON characteristic decreases.
As described above, in a zero-cross type photo-trigger thyristor exists a specific characteristic representing the maximum VAK voltage build-up rate for the photo-trigger thyristor to be turned on at the zero-cross portion, i.e., the dV/dt ON characteristic. It is desirable that the value of the dV/dt ON characteristic be as large as possible, and the voltage VW is wanted to be larger. In contrast, in order to improve the dV/dt withstandability of the photo-trigger thyristor, it is desirable to turn on the MOSFET by as small a voltage VAK as possible with a contradictory request to make the value of the dV/dt ON characteristic smaller. Conventionally, however, it is difficult to satisfy both a large value of the dV/dt ON characteristic and a high dV/dt withstandability.
Accordingly, it is an object of the present invention to provide a photo semiconductor device with a structure which can improve the dV/dt ON characteristic without reducing the dV/dt withstandability in a photo-trigger thyristor having a zero-cross circuit using a MOSFET.